Chapter 6 - The Structure of the Solar System With Doctor Bones aka Don R. Mueller, Ph.D.

1. Astonishing Astronomy 101
With Doctor Bones (Don R. Mueller, Ph.D.)
Educator
Entertainer
J
U
G
G
L
E
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Scientist
Science
Explorer

2. Chapter 6
The Structure of the Solar System

3. Components of the Solar System
• The vast majority of the Solar
System’s mass resides in the Sun.
• All the planets, asteroids and comets
make up less than 1/700 of the mass
of the Solar System.
• The rocky inner planets: Mercury,
Venus, Earth and Mars are called
terrestrial planets.
• The gaseous outer planets: Jupiter,
Saturn, Uranus and Neptune are the
Jovian planets.
• An asteroid belt separates the
inner and outer planets.
• Pluto, once a planet, has been
reclassified as a dwarf planet.
Please insert
figure 32.1

4. The Role of Mass and Radius
• Mass and size of a
planet help determine
its environment.
• Small planets cool
quickly, leading to dead
worlds.
• Small planets also have
trouble holding an
atmosphere.
• Larger planets cool
slower, and have active
interiors and surfaces.
• Mars is right in the
middle – not too large,
and not too small.

5. Interiors, Atmospheres and Surfaces of Terrestrial Planets

6. The Role of Water and Biological Processes
• The presence or absence of
water helps determine the
nature of the atmosphere.
• Water acts as a sink for CO2,
removing the greenhouse gas
from the atmosphere.
• Water helps lock CO2 into rock.
• Too much CO2 can lead to a
runaway greenhouse effect
(e.g., Venus).
• Too little CO2 can lead to cooling
(e.g., Mars).
• Biological activity impacts
the environment as well.
• Burning fossil fuels releases
CO2 into the air.
• Animals remove oxygen
from the system and
release CO2 and methane.
• Our planet’s O2 comes from
the breakdown of water
and CO2 by plants.

7. The Role of Sunlight
• A planet’s distance from the
Sun determines how much
sunlight it receives.
• Venus receives ¼ of the
energy per square meter that
Mercury does.
• Planets in eccentric orbits
receive varying amounts of
sunlight.
• The axial tilt of a planet
determines its seasons.
• Sunlight warms, however the
atmosphere has an impact too:
– Venus’s atmosphere warms the
surface to 750 K, but it would be
very warm even without the CO2.
– Mercury is closer to the Sun, but
still cooler than Venus.
– The Moon is cooler than the
Earth, even though they are at the
same distance from the Sun.
• Sunlight also determines the
makeup of the planets:
– Inner planets are rock and iron
bodies.
– Outer planets are gaseous.

8. The Outer Planets
• Far from the Sun: cold enough
that water vapor condenses into
ice.
• Beyond this frost line, planets are
primarily composed of hydrogen
and ices.
• The low temperatures allowed
the inner planets to capture
hydrogen and helium gas and to
grow to immense sizes.
• The outer planets have no
surfaces:
– Pressures steadily climb,
turning gases into liquids and
eventually metals.

9. Equatorial Bulges
• Jovian planets rotate
much faster than
terrestrial planets.
• From the principle of
conservation of
angular momentum.
• Faster rotational
speeds make the outer
planets much wider at
the equator. This leads
to the so-called
equatorial bulge.

10. Other Interesting Differences
• Each gas giant has a
set of rings:
• Easy to see: Saturn
• Hard to see: Neptune
• Gas giants generate
more internal heat
than they receive
from the Sun.
• The gas giants have
many more moons.

11. Differences Among the Giants
• Strong color differences
between the giants are
related to their distances
from the Sun.
• Ammonia and methane
condense at lower
temperatures than water,
so the chemistry of the
outer giants differs from
the inner giants.
• The least massive of the
giants (Uranus) also seems
to generate the least
internal heat, again similar
to the terrestrial planets.

12. The Kuiper Belt
Outside the orbit of
Neptune lies the
Kuiper Belt.
Located around 40
AU from the Sun:
Trans-Neptunian
Objects (TNOs)
such as Pluto are
found here.
Bodies smaller and
larger than Pluto
are in this region,
including the
dwarf planet, Eris.

13. Pluto’s Reclassification:
Will the “Real” Pluto please stand up.
• In 1920, Pluto was discovered and
classified as a planet.
• Is Pluto a planet? The debate:
A planet must be massive enough:
(1) for its gravity to pull it into a roughly
spherical shape, and
(2) for it to have cleared out the
neighborhood of its orbit of
comparable mass objects.
• This means that the objects lying
in both the asteroid and Kuiper
belts are not planets.
• Alas, in 2006, Pluto was
reclassified as a dwarf planet.
Pluto?
1920
versus
Pluto?
1930

14. Opik – Oort Cloud
Ernst Julius Öpik (1893 –1985) was an Estonian astronomer.
Jan Hendrik Oort (1900 – 1992) was a Dutch astronomer .
• The Solar System is
surrounded by a
cloud of cometary
bodies:
– Located around
50,000 AU from
the Sun.
– Gravitational
influences from
passing stars
occasionally send
comets into the
Solar System.
Please insert figure 32.3

15. Rotation and Revolution in the Solar System
http://www.youtube.com/watch?v=9R5P9Y9gRYY&feature=related
• Due to the conservation of angular momentum, all planets revolve
around the Sun in the same direction and nearly the same plane:
– Mercury’s orbit is tipped by 7 degrees.
• Most of the planets rotate in the same direction:
– Counterclockwise as viewed from above
– Venus rotates clockwise as viewed from above
– Uranus’ rotational axis is tipped significantly
Orbits of all the planets (Including Comets)
http://www.youtube.com/watch?v=NrODEmei-wA&feature=related
The comet Shoemaker-Levy, discovered in 1993, was
important because it was the first comet humans
witnessed impacting a planet.

16. Planetary Tilt Angles

17. Calculating a Planet’s Density
• Calculate the planet’s mass (M)
by observing its satellite’s orbital
distance (d) and period (P)
• Use Newton’s modified form of
Kepler’s 3rd Law:
• If we know the distance to the
planet, we can measure its
angular diameter and calculate its
linear diameter (radius) and then
its volume:
• The planet’s average density  is
then:
2
3
GP
πd4
M 
3
πR
3
4
V 
V
M
ρ 

18. verage Densities of the Planets in our Solar System
nner planets have high average densities (~5 kg/liter): Small bodies of rock and iron.
uter planets have lower densities (~1 kg/liter): Large bodies of gas and ice.

19. The Age of the Solar System
Example: Potassium–Argon dating or K-Ar dating
• A number of naturally occurring
atoms undergo radioactive decay.
The time it takes for half of the atoms in a
given sample to decay is called the
material’s half-life.
After n half-lives, the fraction of original
material is:
• We can then use radioactive dating
to determine the age of rocks.
The oldest Earth rocks: 4 billion years old.
Older samples have been found on the Moon
and in meteorites.
• Bodies in the Solar System whose
ages have so far been determined are
consistent with having formed about
4.5 billion years ago.
n







2
1
Fraction

20. Formation of the Solar System: Solar Nebula Theory
• The most successful model of Solar
System formation is the Solar Nebula
Theory:
– The Solar System originated from a
rotating, disk-shaped cloud of gas
and dust, with the outer part of the
disk becoming the planets, and the
inner part becoming the Sun.
• 4.5 billion years ago, the cloud of gas
and dust that would become our Solar
System began to contract.
– Contracting and flattening into a disk
that began to spin faster:
(Conservation of Angular Momentum)
– Most of the material in the cloud
moved to the center to become the
Sun.

21. Planetesimal Formation: From the hypothesis of Viktor Safronov:
Stating that planets formed out of dust grains, colliding and sticking
together to form larger and larger bodies.
• The inner solar system: silicate crystals and metal grains accreted
over time, to form rocky planetesimals: The terrestrial planets.
• In the outer solar system, icy planetesimals formed.

22. Condensation Temperatures of Major Elements
Element Condensation
Temperature (K)
Percent by
Mass in Sun
Percent by Mass
in Earth
Hydrogen 180 (H2O) 70.6 0.0033
Helium 3 27.4 0.00000002
Carbon 80 (CH4) 0.31 0.045
Nitrogen 130 (NH3) 0.11 0.0004
Oxygen 1300 (silicates),
180 (H2O)
0.96 30.1
Neon 9 0.18 0.0000000004
Silicon 1300 (silicates) 0.07 15.1
Iron 1400 0.18 32.1

23. Protoplanets and differentiation
• Planetesimals grew
into protoplanets:
heated by collisions
and by radioactive
decay.
• Denser material sank
toward the center of
the bodies and lighter
material floated
toward the surface.
• This separation
process is called
differentiation.

24. Atmospheric Retention
• Retaining an atmosphere can be a problem.
• Small planets will have low escape velocities.
• Atmospheres around planets close to the Sun
will be very warm, giving the gas atoms a high
thermal velocity.
• If the thermal velocity of atmospheric gases is
close to the escape speed for the planet, the
atmosphere can escape into space.

25. We are Stardust• A supernova or stellar explosion creates an
incredibly luminous burst of radiation that
can outshine an entire galaxy before fading
from view. In this short time interval, the
supernova can radiate as much energy as
our Sun is expected to emit during its life
span. The explosion expels the stellar
material at velocities approaching that of
10% of the speed of light. The shock wave
created sweeps out an expanding shell of
gas and dust called a supernova remnant.
Supernovae, play a critical role in enriching
interstellar media with higher mass
elements. The heavy elements greater
than iron that you are made of were
formed in a supernova.
We truly are “stardust.”

26. The Asteroid Belt: Most asteroids can be found between
the orbits of Mars and Jupiter.
• Using Bode’s Rule (a simple mathematical
formula) the asteroid Ceres was
discovered between the orbits of Jupiter
and Mars

27. The Shapes and Sizes of Asteroids
• Asteroids come in all shapes
and sizes: Big and small.
• Ceres is massive: Large
enough to pull itself into a
sphere and therefore be
classified as a Dwarf planet.
• Most asteroids are small:
tens of kilometers across.
• Still large enough to cause
tremendous damage if
impacting the Earth.
• Spacecraft have only
recently visited asteroids.
Vesta
Eros
Ceres

28. Asteroid Eros: Potato-shaped

29. Visiting Comets
Comet Halley
visited by
Giotto
Comet Wild 2:
visited by
Stardust
Comet Tempel 1:
visited by
Deep Impact

30. The Origin of Comets
• Comets may originate in
either the Oort Cloud or
the Kuiper Belt.
• Oort cloud: a cloud of
comet-like planetesimals
more than 100,000 AU
from the Sun.
• Oort cloud objects may
have formed near the
giant planets and then
were tossed outwards by
gravitational forces.
• Passing stars or other
gravitational influences
nudge the comets into
the inner Solar System.
Please insert figure 47.5